Engineering of PEM-PEC photocathodes for solar-driven hydrogen production

Earth has always been in perpetual evolution, but today we must face its rapid change due to human activity. The intensification of industrial activities and transportation to support our modern lifestyles are the main causes of climate change and the adverse effects on the fauna and flora. In spite of this, the planet has shown resilience and should be able, if the conditions allow, to maintain its balance. Our energy system is at the heart of the transformation we must undertake. To lessen our impact on the environment, we must consider the end of fossil fuels, the main source of greenhouse gas emissions. The immense amount of solar energy which reaches the Earth's surface would be more than enough to meet the world's energy needs, provided that we master its capture and overcome its intermittency. The storage of this energy in solar fuel such as hydrogen is a solution to meet the challenge of intermittent light. Hydrogen can be produced by splitting water molecules into hydrogen and oxygen under the effect of an electric current. Various methods such as proton exchange membrane (PEM) electrolysis can be coupled with solar energy conversion systems to produce carbon-free hydrogen. These technologies are still in their infancy; therefore, their evaluation is a key issue for future energy challenges.The research work presented in this thesis deals with the engineering of a proton exchange membrane photoelectrochemical cell (PEM-PEC) to produce hydrogen from moist air and solar energy. A new porous electrode support combining transparency and electrical conduction has been created. The use of this novel transparent, porous, conductive support (TPCS) for photoelectrode was demonstrated by depositing an n-type semiconductor (hematite, a-Fe2O3) to produce oxygen by solar-assisted water splitting. The hematite-TPCS electrode exhibited a photocurrent of 1.6 mA.cm-2 at 1.6 V vs. RHE (reversible hydrogen electrode). For the photocathode portion different p-type semiconductor layers were studied; first on flat conductive glass and later on the TPCS. Tungsten diselenide (WSe2) nanoflakes produced by liquid phase exfoliation were coated by electrophoretic deposition. Furthermore, an in-situ electro-conversion to form copper oxide (Cu2O) was identified, allowing for production of thin and transparent layers. Subsequently, p-type semiconductor layers (WSe2, Cu2O, organic semiconductors) were deposited on the TPCSs and their photoactivity in liquid media was evaluated. The organic semiconductor was selected based on the ease of implementation and the promising results obtained for the photoreduction of europium in liquid phase, ca. -4.5 mA.cm-2 at 0 V vs. RHE. After Pt catalyst coating, the organic semiconductor based TPCS was tested for hydrogen production in liquid media, exhibiting a photocurrent ca. -1 mA.cm-2 at 0 V vs. RHE. After the implementation in a half PEM-PEC cell for hydrogen production in the gas phase, a photocurrent of -120 µA.cm-2 at 0 V vs. RHE was sustained (i.e., 1 µmol.h-1of H2) for 1 hour.